The effect of nitric acid uptake on the

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May 31, 2002 - HNO3 dissolution in ammoniated aerosol solutions can prevent the efflorescence of ... (NH4)3H(SO4)2 and one mole of NH4NO3. [5] Below we ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 10.1029/2002GL015251, 2002

The effect of nitric acid uptake on the deliquescence and efflorescence of binary ammoniated salts in the upper troposphere Jin-Sheng Lin and Azadeh Tabazadeh NASA Ames Research Center, Moffett Field, California, USA Received 2 April 2002; revised 21 April 2002; accepted 3 May 2002; published 31 May 2002.

[1] Thermodynamic equilibrium model calculations are used to show that HNO3 can lower the relative humidity barrier to deliquesce binary ammoniated salts in the upper troposphere, particularly in aerosol solutions with an (NH4)2SO4 composition. Also in the case of (NH4)2SO4 or NH4HSO4 there is no abrupt change in the physical state of the thermodynamic system (instantaneous change of a dry salt particle into an aqueous solution) at the deliquescent point, which is typical for such a phase transition. Model results show that HNO3 dissolution in ammoniated aerosol solutions can prevent the efflorescence of (NH4)2SO4 and NH4HSO4 salts in favor of (NH4)3H(SO4)2 (letovicite) crystallization. Future laboratory experiments, where binary ammoniated salts are studied at cold temperatures in the presence of both HNO3 and H2O, are needed to confirm the results of our model calculations. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and Particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud Physics and Chemistry; 0399 Atmospheric Composition and Structure: General or Miscellaneous

1. Introduction [2] Aerosols impact the Earth’s climate and chemistry [Ravishankara, 1997]. Both the ice nucleating properties and heterogeneous reactivity of aerosols in the atmosphere are affected by their solution phase and composition. A comprehensive data set on the chemical composition and phase of aerosols in the upper troposphere is currently lacking. Observations from limited aircraft data indicate that comparable amounts of ammonium and sulfate ions may be present in upper tropospheric aerosol solutions [Talbot et al., 1996, 1998]. In most bulk phase aircraft filter samples the ammonium ion mixing ratio in the aerosol is often shown to be sufficient to fully or partially neutralize the sulfate ion [Tabazadeh et al., 1998]. [3] Up to now the deliquescence and efflorescence relative humidities of binary ammoniated salts have been measured in the laboratory in the absence of gas phase nitric acid [Imre et al., 1997; Xu et al., 1998; Onasch et al., 1999; Cziczo and Abbatt, 1999]. Thus far only Brooks et al. [2002] have shown that mixtures of ammonium sulfate and organic acids can deliquesce at lower relative humidities than pure ammonium sulfate. Similarly, here we use a thermodynamic electrolyte model [Clegg et al., 1998] to examine how HNO3, which is abundant in the atmosphere, can affect the deliquescence and efflorescence properties of binary salts in the upper troposphere.

2. Nitric Acid Uptake by Binary Ammoniated Salts 2.1. Deliquescence [4] Traditionally the deliquescence relative humidity (DRH) of a binary salt is defined as the relative humidity at which a dry salt Copyright 2002 by the American Geophysical Union. 0094-8276/02/2002GL015251$05.00

particle instantaneously turns into an aqueous solution droplet. In fact many laboratory observations show that binary salts do abruptly change phase at a fixed RH, known as the DRH [Tang, 1996; Imre et al., 1997; Xu et al., 1998; Onasch et al., 1999; Cziczo and Abbatt, 1999]. However, in the atmosphere such a definition is too simple for two reasons. First, in the atmosphere a dry salt particle is exposed to other gaseous components, besides just H2O which determines the RH. For example, in the case of HNO3, we have shown that it is highly soluble in aqueous (NH4)2SO4 at cold temperatures [Lin and Tabazadeh, 2001]. Also, H+ ions from the HNO3 dissolution process can interact in an (NH4)2SO4 solution to produce (NH4)3H(SO4)2 (letovicite) [Tabazadeh and Toon, 1998]. Second, at low temperatures, the equilibrium composition of a dry binary salt at low relative humidity may be different than that of a pure salt, which may exist at room temperature. For example, the thermodynamic model of Clegg et al. [1998] predicts that at low temperatures (